I think she realized it when she found out her cousin Lola was getting married. At that point she figured it out that Robbie was not Lola's attacker and Lola was marrying him. I'm not sure, but I think Lola may have known who it was and let Briony go with her story. She mentioned that they encouraged her durning the trial. Maybe I read it wrong. I was a little confused at times because I skimmed through some of the wordy parts.